The present disclosure relates to high performance geosynthetic articles, such as reinforcing strips, reinforcing elements, membranes and especially dimensionally stable cellular confinement systems. The present disclosure particularly relates to geosynthetic articles, characterized by structure and composition adapted to provide enhanced properties, formed of or comprising a compatibilized polymeric composition.
Plastic geotechnical reinforcing elements and articles, especially cellular confinement systems (CCSs) are used to increase the load bearing capacity, stability and erosion resistance of geotechnical reinforced materials (refer hereinafter as GRM) such as soil, rock, sand, stone, peat, clay, concrete, aggregate, road building materials, and earthen materials which are supported by the CCS.
CCSs are mostly manufactured from strips that include high density polyethylene (HDPE) or medium density polyethylene (MDPE), and are characterized by a honeycomb-like three dimensional cellular structure. The structure, when filled with a geotechnical material including, for example, soil, rock, sand, stone, peat, and clay, concrete, aggregate, road building and other earthen materials, or mixtures of these and/or other materials, such as fluids contained in the materials, provides reinforcement and stabilization both to the geotechnical materials and surrounding structures.
The CCS strengthens the GRM by increasing its shear strength and stiffness as a result of the hoop strength of the cell walls, the passive resistance of adjacent cells, and friction between the CCS and GRM. Under load, the CCS generates powerful lateral confinement forces and soil-cell wall friction. These mechanisms create a bridging structure with high flexural strength and stiffness. The bridging action improves the long-term load-deformation performance of common granular fill materials and allows dramatic reductions of up to 50% in the thickness and weight of structural support elements. CCSs may be used in load support applications such as road base stabilization, intermodal yards, under railroad tracks to stabilize track ballast, retaining walls, to protect GRM or vegetation, and on slopes and channels.
The term “HDPE” refers hereinafter to a polyethylene resin characterized by density from 0.941 to at least 0.960 g/cm3, and the term “MDPE” refers to a polyethylene resin characterized by density from 0.926 to 0.940 g/cm3.
The reinforced CCS is a composite structure, wherein the GRM is compressed and densified against the CCS walls and the friction between walls and GRM keep the integrity of the structure. The plastic cells and the GRM infill dynamically support each other and should be able to survive under a wide spectrum of loads, vibrations, impact loads, thermal stresses and erosion.
Three major factors affecting the long term effective durability of GRM-CCS composite structures are (1) the creep resistance of the plastic material; (2) the friction between cell wall and the geotechnical reinforced material (GRM) which is stabilized and reinforced within the CCS; and (3) the dimensional stability of the compacted GRM and the CCS.
Creep of the CCS wall causes loosening of the friction and loss of structural functionality of the CCS-GRM composite structure. HDPE and other polyolefins fail to resist creep, especially at temperatures higher than about 35-40 degrees Celsius (° C.). The situation with MDPE is even worse.
The potential to heat geotechnical articles, and particularly CCS, is usually correlated with hot areas on the globe. As used herein, in one embodiment, the term “hot areas” refers to areas located within 42 degrees latitude on either side (north or south) of the Earth's equator. In one embodiment, “hot areas” refers to areas located within 30 degrees latitude on either side of the Earth's equator. In particular, hot areas include regions along and in the desert belt. For example, North Africa, Southern Spain, Middle East, Arizona, Texas, Louisiana, Florida, Central America, Brazil, India, southern China, Australia and part of Japan may be considered hot areas. In general, such hot areas regularly experience temperatures above 35° C., or even above 40° C. Surfaces of plastic articles exposed to direct sun light may reach temperatures of 75° C. and even up to 90° C.
The mechanism of failure of CCS at elevated temperatures can be complex. The first step is the heating of the GRM surface and the exposed CCS surfaces—especially by absorption of sunlight. The increase of CCS temperature causes a dimensional change, since polyethylene (PE) has a high coefficient of thermal expansion (CTE)—about 150-200 parts per million per degree C. temperature change (ppm/° C.) and the CTE itself actually increases with increasing temperature. This means that 100 meters (m) of CCS will increase its length by about 60-80 centimeters (cm) when heating from 25° C. to 65° C. Since the GRM held by the CCS expands much less, the coupling between GRM and CCS, i.e., the ability of the CCS to hold the GRM, is deteriorated. In addition, when the CCS is exposed to heat for a few hours per day, the exposure leads to creep and irreversible expansion. The result is that even when temperatures fall, the intimate contact between the CCS and the GRM is irreversibly reduced and thus the combined structural integrity and performance are reduced or even lost. Thus, the repeated cycles of heating, expansion of the CCS, resultant spreading or collapse of the GRM structure previously contained by the CCS, results in eventual failure or significant loss of function of the CCS.
The situation becomes worse when the GRM is subjected to freezing and thawing of water during autumn and winter, a process that causes expansion of the GRM against the CCS. Since the creep resistance of HDPE and MDPE is medium or even low, the result is further loss of contact between CCS walls and GRM. This process is naturally occurring, for example a process in which stones are pushed up out of soil in winter, due to the cycles of freezing and thawing or, for example, freezing of water in cracked concrete or rock, leading to breaking of the concrete or rock.
The shear surfaces of the CCS structure and usually the walls of each cell, may be embossed, or provided with other means of friction-enhancement, in order to increase friction with GRM and prevent deformations of the wall so that the integrity of the composite structure is not ruptured.
Commercially available HDPE-based CCSs are characterized by moderate stiffness, moderate dimensional stability and acceptable creep resistance at temperatures in the range of about minus 10° to plus 40° C. These CCSs are however characterized by some drawbacks: They have moderate strength, high CTE, high tendency to creep, especially when temperatures are 40° C. and over, and chemical sensitivity to hydrocarbons and more specifically fuels and oils.
Chemical sensitivity to hydrocarbons is deleterious for applications wherein the CCS or membrane is subjected to fuels and oils, for example as GRM reinforcement or for confinement in landfills, oilfields, gas stations, intensive parking areas and chemical industry or as a barrier in landfills and reservoirs.
The limiting mechanical and chemical properties of HDPE and MDPE, and of course other polyolefins, are especially pertinent to creep resistance and limited thermal resistance as well as a high tendency to swell when exposed to hydrocarbon fluids. If one compares creep resistance and chemical resistance to hydrocarbon fluids, under the same load, between engineering thermoplastics (“ET”), such as between polyamide or polyester on one hand, and polyurethanes on the other, the engineering thermoplastic resin is by far more dimensionally stable, stiff, has a much lower tendency to creep, has much higher chemical resistance against fuels and organic fluids and higher strength.
In contrast to the ETs, polyethylene has better tear and puncture resistance than engineering thermoplastic—especially at temperatures below zero degrees Celsius. Tear strength and puncture resistance are important issues in membranes and CCSs, and even more important in perforated CCS wherein perforation provides drainage through the plastic wall, but weakens the strip and increases its sensitivity to tearing. Tear and puncture resistance is also important during installation wherein the CCS is still empty before filling with GRM, and needs to survive human activity related to installation and GRM filling.
The advantages of the engineering thermoplastic are even greater when properties are compared at temperatures above about 40-50° C. Since most CCS are manufactured by welding of a plurality of strips, the welding strength and rate of weld formation is better with engineering thermoplastic relative to HDPE or MDPE. Another advantage of engineering thermoplastic-based CCSs is the improved coefficient of friction with GRM and especially with soils and peat, compared with polyolefins, due to their higher polarity. Engineering thermoplastics are also more resistant against swelling by hydrocarbons such as fuels and oils.
The major limiting factors of engineering thermoplastics as the resin of choice in manufacturing of CCS, are a high modulus of elasticity, which affects installation simplicity, relative high cost, relatively higher sensitivity to acids and bases, relative brittleness at temperature below about 10° C. and a low melt strength that affects the simplicity of strip extrusion.
The combination of engineering thermoplastic resins and polyolefin resins in one blend, is described in several prior art patents.
U.S. Pat. No. 3,963,799 provides compositions of polyamide and polyolefin, adapted mostly for packaging industry and methods to form alloys (compatibilized blends) thereof. The compositions described in this patent are not applicable for structural geotechnical applications including CCSs, due to its inherent brittleness, especially at low temperatures, and lack of protection against humidity and UV light. This patent does not deal with either the difficulties in welding of the compositions, or the hydrolytic instability of the polyamide phase, which may be hydrolyzed in soil, especially acidic soils. Moreover, the compositions of this patent have CTE too high for CCS and membranes.
U.S. Pat. No. 4,564,658 provides compositions of polyester and linear low density polyethylene (LLDPE) only, and provides no compatibilizer, i.e., no agent to stabilize the dispersion of the two immiscible polymers. Consequently, in extrusion applications, for example extrusion of strips for geotechnical applications, flow of the melt is uneven (melt fracturing), and segregation between phases is observed. The compositions described in this patent are not applicable for structural geotechnical applications including CCSs, due to their flexibility and creep tendency. Due to the nature of LLDPE the compositions of this patent have CTE too high for CCS and membranes.
The patent also does not provide a solution for the protection of the blend from hydrolysis in soils and landfills, oils and hydrocarbons, and from the degradation induced by heat and UV light. Welding quality is not discussed.
U.S. Pat. No. 5,280,066 provides compositions of polyester, polyolefin and a functionalized styrenic elastomer for improved impact resistance, especially for injection molding. The invention is limited only to polypropylene (PP) as the polyolefin fraction. PP is too rigid and lacks the flexibility at temperatures below about zero ° C., a property that is mandatory in CCSs. The compositions of this patent have CTE too high for CCS and membranes.
The compatibilizer according to this patent is styrene based—thus has limited UV light resistance and thus limits the composition to indoor application or outdoor application with a lifetime of about 1 to 2 years. Polyester blends, especially when not specially stabilized against hydrolysis, may fail in soils, especially those having pH greater than 7, within a relatively short period of time. This patent does not provide sufficient protection against oils and fuels, acids and bases and UV light. Welding quality is not discussed.
U.S. Pat. No. 6,875,520 provides compositions of polyamide block copolymer and a very flexible polyolefin. This invention may be useful for flexible geomembranes but not for structural geotechnical applications including CCS and high performance membranes. The high flexibility that is an advantage in flexible geomembranes becomes a drawback in CCS: when a load is applied on the CCS supporting GRM, the composite structure of the two components interacts with the load as an integrated system. The CCS transfers the load from cell to cell by friction with the GRM which provides rigidity and stiffness. If the CCS is too flexible, the load induces a deformation of the CCS until friction with the GRM is lowered. At that specific state, the integrated system is irreversibly damaged and can no longer provide the required durability, stiffness and stabilization to the GRM. The patent does not provide a solution to the hydrolysis of the composition in soils and landfills, or when exposed to concrete or other media characterized by pH of greater than 7. The compositions of this patent have CTE too high for CCS and membranes.
UV and heat stability, especially for extended periods of 2 years or more, that are required from CCSs, are not discussed or provided as well.
There thus exists a long felt need to provide an improved polymeric composition, particularly as compared to HDPE and MDPE, characterized by having improved properties, such as one or more of creep resistance at a wide range of temperatures, such as temperatures in the range of minus 70° to plus 90° C., being stronger and stiffer, having lower CTE and lower tendency to lose its stiffness at elevated temperatures, having higher resistance to creep during freeze/thaw/heating of GRM, being more resistant to swelling by low molecular weight materials such as oils and hydrocarbons, having greater resistance to UV light and thermal degradation for periods of about 2 to about 100 years in a wide spectrum of climates ranging from arid to arctic and having improved welding strength and weld load bearing resistance. Such improved polymeric compositions would be desirable for CCS for high performance applications and for reinforcing GRM comprising oils, acids and bases, aggressive chemicals, solvents and fuels. In addition there is a need for improved geotechnical articles such as geomembranes and geogrids, having improved properties important to the applications to which such articles are put. The need for such compositions and materials made therefrom has remained unmet until now.